The selenium moiety of the glycine reductase selenoprotein was isolated from Se75-labeled protein after alkylation with several different alkylating agents and identified as a selenocysteine residue. This is the first instance in which an essential organoselenium compound in a protein has been identified. The formate dehydrogenase, as well as the glycine reductase of C. sticklandii, was shown to be a selenoprotein and thus two selenium-dependent enzymes produced by the same organsism are available for comparative studies. Studies on further purification and characterization of the glycine reductase enzyme system, the formate dehydrogenases and the methane forming system have been continued in order to further understand these anaerobic oxidation-reduction systems.